1. The Field of the Invention
The present invention relates to a method for reducing NOx emissions during combustion of coal, more particularly to methods that utilize coal that has been cleaned to lower the ash while reducing the ratio of primary combustion air to fuel and compensating for such reduction with increased secondary and/or tertiary (or overfire) air.
2. Related Technology
Coal combustion is a major source of energy for the production of electricity throughout the world. Coal is a good source of energy because of its high energy to weight ratio and its great abundance. The use of coal, however, is increasingly under scrutiny because of environmental concerns. Among the known environmental difficulties with coal combustion is the production and emission of NOx compounds, such as NO, N2O, and NO2. NOx compounds can be very harmful to human health and are known to produce undesirable environmental effects such as smog and acid rain.
Government regulations require emission from coal burning to be monitored and controlled. Controlling NOx emissions has become increasingly important as government regulations continue to lower the allowable level of NOx and other pollutants that can be released into the environment. The requirement for reduced pollutants from coal-fired power plants has led to a demand for suitable new technologies.
In a coal fired power plant, there are two principle sources of NOx formation: fuel NOx and thermal NOx. Fuel NOx is formed from bound or fixed nitrogen contained in the fuel, whereas thermal NOx is formed from non-fuel sources of nitrogen, such as nitrogen contained in the combustion air. About 80% of NOx emissions from coal combustion are produced from fuel nitrogen.
One method used to reduce pollutants during coal combustion focuses on removing NOx from power plant flue gas. For example, NOx emitted in flue gas can be removed using selective catalytic reduction (SCR), which converts NOx compounds to nitrogen gas (N2) and water. However, this type of NOx control method is expensive, in part, because of the required capital investment. The cost of these technologies and increasingly stringent government regulations have created a need for less expensive technologies to reduce NOx emissions from coal combustion.
Another method of reducing NOx emissions is to remove coal nitrogen from the coal material by converting it to N2. Researchers have discovered that iron-based catalysts can assist in releasing fuel nitrogen from coal. Ohtsuka and coworkers at Tohoku University (Sendai, Japan) describe methods for producing an iron-based catalyst which, when combined with coal and placed in an pyrolysis environment, causes nitrogen compounds in coal to be released more rapidly, thus causing a decrease in the amount of nitrogen remaining in the char material (Ohtsuka et al., Energy and Fuels 7 (1993) 1095 and Ohtsuka et al., Energy and Fuels 12 (1998) 1356). Such methods for reducing NOx have been impractical. Ohtsuka precipitates FeCl3 solution directly onto coal using Ca(OH)2, which results in an increase in the ash content (up to 7 wt % iron) and requires washing with water to remove chloride salts, thus also adding water to the coal.
Improvements to the Ohtsuka method are disclosed in U.S. Pat. No. 7,357,903, entitled “METHOD FOR REDUCING NOx DURING COMBUSTION OF COAL IN A BURNER” and assigned to Headwaters Heavy Oil, LLC of South Jordan, Utah. This patent discloses applying a nanoparticle catalyst to coal, either before or after it is pulverized, and then burning the treated coal in the low oxygen zone of a coal burner. Preliminary tests showed that the nanoparticle catalyst was effective in reducing NOx formation, presumably by catalyzing more rapid release of fuel nitrogen from the coal to form N2 in the low oxygen zone before it can combust to form NOx in more oxygen rich zones of the burner.
In general, determining how much NOx a particular coal will produce during combustion can be a very difficult calculation; NOx emissions can be highly variant depending on the combustion temperature, oxygen level, and fuel nitrogen content.
In view of the foregoing, there remains a need to find improved coal treatment and combustion methods for reducing NOx emissions, particularly methods that can utilize a wide range of different types of coal feedstocks having greatly varying quality while reliably reducing NOx emissions compared to conventional methods.